IIRC you have to be on the topmost level of your circuit to be able to move the controls onto the panel.
So try this:
- Goto to the topmost level in Circuit mode
- Switch to Surface mode and Move mode
- Drag the control to the panel

I can't see any custom size and no auto. And also no GO Editor.
If you refer to 'GO' Tree: yes this exists, but only the 'visible' and 'position origion' check do anything. If I change 'type' the panel vanishes for good.

These interact. I'll explain how to use them:
1-Make sure you have your project onscreen, so viewing all the way up, not looking into devices. You should also see the in and outputs of the card/xite.
2-Enter your device/doubleclick on it. You will now see your circuits and you should also see a SurfaceInterface module connected to Panel module, aka your main GUI.
3-Don't enter any sub modules, but just switch from circuit mode to surface mode. You should now see your main GUI on screen!
4-Select(not enter) the main panel object/background and open your GO-Tree window; it will show a panel folder, with a ClientArea subfolder inside.
5-See the attached screenshot of such a ViewTree. You see that the Panel folder(GO-Tree), is a group containing your ClientArea. In the actual module, as shown in your Project explorer will have more elements such as knobs, faders, controllers, text etc. You can also group elements or objects on your GUI for keeping them together while moving for example. Groups can be very handy. In Surface mode They are the equivalent of folders in the Circuit mode.
6-When things are grouped, certain flags on the GO-Tree editor are set by default. To cut things short: while you are developing your device and the GUI is not finished, the flags on a group, and also the main panel one!: VTG should be enabled, and the CHS and CVS should be enabled.

The ViewTree option, makes the group as large as is needed to included ALL it's siblings. Also the ones which are placed outside a panel, or you placed it outside the group when it didn't have the correct dimensions! (this is how you lose controls from screen when moving them around when the size is fixed and things are clipped. I'll come back to this later on)

So while developing: VTG on, CHS and CVS off.
When your finished: Watch order!!!!! CHS and CVS on, and then VTG off. NOT the other way around!

The CHS and CVS will be set to current size, including the siblings/elements of the group.

Now: Select Clipping, will make sure a user cannot select outside that group's dimensions. It is possible to place things outside a group after the size has been set to custom. That is....if DrawClipping isn't enabled. DrawClipping will make sure that elements outside the groups dimension are not drawn onto screen, they are invisible (if size is at custom). You CAN enter any group, and the elements hidden or outside a group can be seen after entering a group. This is all done in surface mode!

When you finalize your device, you should first check and optimize. Probably you will get a lot of ViewTree messages. We have all seen them. Make sure you set ALL the groups to correct parameters. So...select a group, enable CHS and CVS and then disable VTG. If you have done that all correct, and thought about the SelectClipping and DrawClipping you should have a fast GUI as far as user experience goes.